Why Don't Catholics think the Bible is the sole rule of faith

                         “Love the Lord with all your heart, and with all your soul, 

                                          and with all your MIND”--Matt 22:37

Question:  Why don't Catholics think the Bible is the sole rule of faith?

Short Answer:  Because the Bible never says that it's the sole rule of faith.  

Longer Answer:  It's a self-refuting doctrine to claim that the Bible is to be used as the sole source of doctrine. Whenever we are approached by a non-Catholic Christian with this question, we should ask, "Where does the Bible claim to be the sole rule of faith?"

Answer: no where.  However, typically what is offered by the non-Catholic is 2 Timothy 3:16:  "All Scripture is inspired by God and useful for teaching, for refutation, for correction and for training in righteousness".

Catholics give a hearty AMEN! to 2 Timothy 3:16...however, saying that all Scripture is inspired isn't the same thing as saying that the Bible is the sole rule of faith.  What 2 Timothy 3:16 means is: if it's in the Bible, we know that it comes from God.

However, non-Catholic Christians still have the onus of providing a Bible verse that says that the Bible is all we need to follow Christ.

Often, they will respond with, "Well, then you Catholics must not view the Bible as the Word of God!"

Nothing could be further from the truth.  Catholics revere the Bible.  As the Catechism states:

For this reason, the Church has always venerated the Scriptures as she venerates the Lord's Body. She never ceases to present to the faithful the bread of life, taken from the one table of God's Word and Christ's Body.

In Sacred Scripture, the Church constantly finds her nourishment and her strength, for she welcomes it not as a human word, "but as what it really is, the word of God". "In the sacred books, the Father who is in heaven comes lovingly to meet his children, and talks with them."

We proclaim that the Word of God is Jesus Christ Himself, and that he was revealed his Word through 2 channels: Scripture and Tradition.  Sacred Scripture is the written Word, and Sacred Tradition is that which was orally transmitted, from Christ, through His Apostles, to the Catholic Church.  (Sacred Tradition is not to be confused with cultural traditions, or customs.  Sacred Tradition has nothing to do with "how we customarily did things--it's just our tradition to do it that way".)

One example of Sacred Tradition is the 27 book canon of the New Testament.  That is, the table of contents.  No Christian can know what books belong in the Bible, except through submitting to the authority of the Catholic Church. There were over 100 different early Christian texts which the early Church Fathers assessed and evaluated, and of those, 27 books were discerned to be the inspired Word of God.  

That is, the Bible didn't tell us what belongs in the Bible.  All Christians--even those who claim to be Sola Scriptura (Bible Alone) advocates--unwittingly defer to Sacred Tradition when it comes to knowing what's actually an inspired text.

Incidentally, when in dialogue with Christians who object to the Catholic view of Sacred Tradition and who hold a Sola Scriptura point of view, it might be helpful to show them that they do indeed believe some things NOT found in the Bible, and thus are contradicting their Sola Scriptura paradigm.

Most Christians believe these things that are not found in the Bible:

-God's Revelation ended with the death of the last apostle.  Not in the Bible, but believed by a majority of Protestants.

-The canon of Scripture is closed.  Christians believe that there are no further books that may be considered inspired.  This is something known by Tradition, but not found in the Bible.

-And, (again, referencing the table of contents of Scripture)--that the Gospel of Mark, for example, is the Word of God. For the Bible does not say that the Gospel of Mark is inspired. (And even if an ancient text did claim inspiration, does claiming to be inspired make it so? Nope. The ONLY way that any Christian knows that the Gospel of Mark is God-breathed is because the Catholic Church told them it is.)

Incidentally, Bible Alone Christians also have a lot of practices that aren't found in the Bible--having a steeple on their building, folding their hands in prayer, Wednesday evening Bible studies, altar calls...so if they're going to be consistent, they should not object to Catholic practices which also aren't found in the Bible, such as praying the Rosary, devotions to particular saints, blessing ourselves with holy water.